The present invention relates to vapor grown carbon fibers having increased bulk density and to the method of increasing such bulk density.
Commercial carbon fibers hold great promise as a high-performance material for composites due to their high strength and high modulus. They are commonly made by elevating a precursor material such as polyacrylonitrile (PAN) or pitch in an inert atmosphere to a temperature around 1000.degree. C. on continuous wind-up devices. They are generally continuous filaments and approximately 8 .mu.m diameter.
However, this application is concerned with vapor grown carbon fibers that are a recent entry in the field of carbon fibers and have similar or even superior physical properties along with the potential for production at a lower cost. Vapor grown carbon fibers are produced directly from hydrocarbons such as methane in a gas phase reaction upon contact with a catalytic metal particle around 1000.degree. C. in a non-oxidizing gas stream. U.S. Pat. No. 5,024,818 to Tibbetts et al. and U.S. Pat. No. 5,374,415 to Alig et al describe typical reaction processes and chambers. Vapor grown carbon fibers differ substantially from commercial carbon fiber in that the fibers are not continuous. The length is around 0.001 to 0.04 mm and the fibers exist as an entangled mass as shown in FIG. 1.
More importantly and pertinent to this application is that the fiber diameter of a vapor grown carbon fiber is generally under 1.mu. and 0.2.mu. is a common average. As those familiar with the growth of vapor grown carbon fibers know, these fibers can be subsequently thickened to the diameter of commercial fibers. However, this can be rather expensive and the resulting fibers are not as graphitic as the original filament graphitization ultimately characterizes the strength and modulus of the fiber. Thus, it is desirable to use fibers that are smaller than the diameter of a commercial fiber by a factor of 40. In terms of surface area, one gram of commercial fiber, which must be considered for coating and bonding to a plastic or rubber matrix, covers about 5.4 ft.sup.2, whereas the surface area of one gram of the entangled mass of vapor grown carbon fibers occupies about 108 ft.sup.2. In other words, 20 times as much coating must be applied to the vapor grown carbon fiber to achieve an equivalent coating thickness. In spite of this obstacle, this presents an opportunity for tremendous potential for rubber and plastic reinforcement.
Further, as these vapor grown carbon fibers are much finer than their continuously grown counterparts, the fibers exist upon leaving the reactor as an entangled mass that is very lightweight with a large apparent volume, from 5 to 50 ft.sup.3 /lb. In other words, the fibers are a lightweight, fluffy entangled mass. In this state, the fibers are very difficult to ship and handle. Such a light and fluffy material is almost impossible to incorporate into mixing equipment that typically processes rubber or plastic. The fly loss and incorporation time is tremendous. Due to a low surface tension, the fibers can not be easily wetted out or mixed into liquid applications without prior surface treatments. These problems represent a severe limitation on the use of vapor grown carbon fibers as they can not be readily dispersed into rubbers, plastics or the like. Thus, the development of methods by which the vapor grown carbon fibers are wet out and densified are key to the commercialization of these materials.
U.S. Pat. No. 5,171,489 to Hirao et al discloses a method for producing composite fibers for electrical conductive applications by combining vapor grown carbon fibers with solid resin particles and heating to form a molten polymer, extruding the mixture from a high speed centrifuge to form filaments, blowing hot gas onto the filaments to form a composite, and subsequently graphitizing by a further heat treatment. However, Hirao et al do not teach a high final bulk density nor do they teach the use of a latex liquid system to enhance adhesion in rubber or like materials. Further, Hirao et al require an additional heat treatment. This additional heat treatment drives off any remaining nitrogen or oxygen groups from the surface of the fibers. Nitrogen and oxygen groups on the surface of the fiber enhance adhesion of subsequent materials with which the fibers are blended.
U.S. Pat. No. 4,855,122 to Kitamura et al discloses a method for increasing the bulk density of carbon fibers. In Kitamura et al, carbon fibers are dipped in an inorganic or organic binder and then dried. The final bulk density of the fibers is from 0.2-0.8 g/cm.sup.3. However, Kitamura et al use carbon fibers from traditional PAN processes which are, in general, much thicker than vapor grown carbon fibers. Further, Kitamura does not blend the fibers and the product has a relatively low final bulk density.
Accordingly, there remains a need for increasing the bulk density of fine carbon fibers and pelletizing such fibers, particularly vapor grown carbon fibers, as well as increasing their adhesion in rubbers, plastics and like materials.